The metallic tube in a hot finished condition is subjected to a cold working process, when the metallic tube does not satisfy requirements in quality, strength, or dimensional accuracy. Generally, examples of the cold working process include a cold drawing process in which a plug or a mandrel and a die are used and a cold rolling process in which a cold pilger mill is used.
In the cold drawing process, a tube end of a mother tube is swaged by a swaging machine; acid pickling is performed to remove a surface scale and the like; and lubricating treatment is performed to draw the mother tube through a die. Examples of the cold drawing process include plug drawing, drawing by using a floating plug, drawing by using a mandrel bar, and sinking drawing without a plug. All the cold drawing processes are performed by diameter reduction working with the die (for example, see “Iron and Steel Handbook third edition” vol. 3, (2) Steel Bar, Steel Tube, and Rolling Common Facilities, pp. 1158 to 1183).
FIG. 1 is an explanatory view showing a diameter reducing drawing process, FIG. 1A shows the plug drawing, and FIG. 1B shows the drawing by using the mandrel bar.
The plug drawing shown in FIG. 1A is a most common drawing process. In the plug drawing, a plug 3, which includes plug support rod 4, is inserted into a mother tube 1, the tube end of the mother tube 1 is gripped with a chuck 6, and the mother tube 1 is drawn through a die 2 in the direction shown by an arrow X in FIG. 1. The plug drawing has advantages in plug exchange and operation efficiency, and also allows a large reduction rate.
The drawing by using the mandrel bar shown in FIG. 1B is a process, in which a mandrel bar 5 is inserted into a mother tube 1 and the mother tube 1 with mandrel bar is drawn through a die 2 like plug drawing shown in FIG. 1A. In the drawing by using the mandrel bar, since the working of tube inner surface is performed by the mandrel bar, a product tube 7 having a glossy inner surface can be produced with high dimensional accuracy even in small diameter tubes. Therefore, the drawing by using the mandrel bar is used in producing a high grade tube for use in nuclear power plants and the like.
Most drawing machines used in the cold drawing are driven by a motor with a chain, but some drawing machines are driven hydraulically using medium of either oil or water.
In the metallic-tube cold drawing process, friction resistance is generated between the outer surface of tube material and the die surface and between the inner surface of tube material and the surface of the plug or mandrel bar, and the drawing is performed against the friction forces. Therefore, tension is generated in a longitudinal direction of tube material. Given tension stress is defined as: tension divided by post-drawing sectional area, when the tension stress becomes high, there occurs a phenomenon that the drawn tube diameter gets unexpectedly smaller, and the tube may rupture in the event that the tension stress reaches a deformation resistance of the tube material. Obviously, the thinner the wall thickness of the tube becomes, the more the tension stress is increased in a longitudinal direction, whereby the tube is ruptured easily. Therefore, there is inevitably a limit to a reduction rate of the wall thickness. Accordingly, in the drawing with the large reduction rate of the wall thickness, it is necessary that the number of drawing be increased to repeat the drawing, and the lubricating work is required in each case, which results in cost increases. In the case where work hardening is significantly generated in the tube material, annealing is also required.